# P1-2: Host Resource Requirement Select — Investigation

## Status
- [>] Under investigation (moved from apiserver-backend-tasks.md)

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## Background

The `select` field in an LSF-style resource requirement string (e.g. `select[mem>4096 && type==X86_64]`)
filters candidate hosts before slot allocation. Currently `EvalSelectForHost` is a no-CGO stub that
always returns `errNoCGO`.

The full feature requires two layers:

1. **C parser + evaluator** — parse the boolean select expression into an AST and evaluate it against
   a host's resource values.
2. **Go bridge** — call the C evaluator via CGO and wire it into `FilterHostByResReqSelect` in the
   controllers layer.

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## Sub-tasks

### #11 P1-2a: C select expression parser (PEG + eval)

**Files to touch:**
- `include/resreq.h` — Replace empty `xce_resreq_select_` stub with expression-tree types, enums,
  and `extern` declarations for `parse_select`, `eval_select`, `free_select`.
- `lib/xce_select.peg` — New PEG grammar for select boolean expressions (right-recursive, handles
  `&&`, `||`, `!`, comparison operators, numeric literals, string literals, resource names).
- `lib/resreq.c` — Implement `parse_select()`, `eval_select()`, `free_select()`; update
  `kill_primary_resreq()` to call `free_select()`.
- `lib/meson.build` — Add `xce_select.peg` to explicit source list so the PEG tool generates C.
- `lib/testsuite/` — TDD tests for parse/eval before implementing.

**Open questions:**
- Which PEG library is already available in the Nix/Meson build? (`peg`, `lemon`, `re2c`?)
- Exact grammar for comparison operators — LSF allows `mem > 4096`, `type == X86_64`, `swp >= 0`.
  Need to confirm all supported operators and value types.
- Does `eval_select` receive a flat key-value map of resource values, or a pointer to the full
  `xce_host_t` struct?
- Error reporting strategy — return a bool+error-string, or a tri-state (true/false/parse-error)?

### #12 P1-2b: Go EvalSelectForHost + FilterHostByResReqSelect (blocked by #11)

**Files to touch:**
- `backend/clib.go` — Implement `EvalSelectForHost(hostname, selectExpr string) error` using
  `cgo.Handle` for callback context; add `#include "resreq.h"`, extern C declarations,
  `xce_select_destroy` helper.
- `controllers/hosts.go` — Implement `FilterHostByResReqSelect(hosts []models.HostInfo, expr string)`,
  update `preCheckHostRequest()` limit from 4094 → 1024 bytes.

**Open questions:**
- Does `EvalSelectForHost` need to look up live resource values from the in-memory store, or are
  values passed in directly by the C layer?
- `cgo.Handle` usage pattern — confirm whether a per-call handle or a session-level handle is needed.
- Input validation: max expression length (1024 bytes), allowed characters.

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## Investigation Checklist

- [ ] Survey existing PEG tooling in the Meson/Nix build environment.
- [ ] Read `include/resreq.h` current state to understand `xce_resreq_select_` stub.
- [ ] Trace how `kill_primary_resreq()` works today and where `free_select()` must be called.
- [ ] Confirm resource value structure passed to `eval_select`.
- [ ] Confirm LSF select grammar spec (operators, value types, nesting depth).
- [ ] Prototype PEG grammar for a subset (numeric comparisons only) and get it compiling.
- [ ] Decide error propagation strategy across the CGO boundary.
- [ ] Write TDD tests for C parser before any implementation.